Fuel cells provide an environmentally friendly source of electrical current. One type of high temperature fuel cell used for generating electrical power is the solid oxide fuel cell (SOFC). The SOFC includes an anode channel for receiving a flow of hydrogen gas (or a fuel gas which reacts in the anode channel to generate hydrogen by steam reforming and water gas shift reactions), a cathode channel for receiving a flow of oxygen gas, and a solid electrolyte which is a ceramic membrane conductive to oxygen ions and separates the anode channel from the cathode channel. Oxygen in the cathode channel dissociates to oxygen ions, which cross the electrolyte to react with hydrogen in the anode channel to generate a flow of electrons. As the hydrogen is consumed, carbon monoxide may be oxidized directly or may be shifted by steam to generate additional hydrogen. Carbon dioxide and water vapor are produced in the anode channel by oxidation of fuel components. Typical operating temperature of solid oxide fuel cells is about 500° to about 1000° C.
Except in the rare instance that hydrogen (e.g. recovered from refinery or chemical process off-gases, or else generated from renewable energy by electrolysis of water) is directly available as fuel, hydrogen must be generated from fossil fuels by an appropriate fuel processing system. For stationary power generation, it is preferred to generate hydrogen from natural gas by steam reforming or partial oxidation to produce “syngas” comprising a mixture of hydrogen, carbon monoxide, carbon dioxide, steam and some unreacted methane. As hydrogen is consumed in the fuel cell anode channel, much of the carbon monoxide reacts with steam by water gas shift to generate more hydrogen and more carbon dioxide. Other carbonaceous feedstocks (e.g. heavier hydrocarbons, coal, or biomass) may also be reacted with oxygen and steam to generate syngas by partial oxidation, gasification or autothermal reforming. The fuel cell may also be operated on hydrogen or syngas that has been generated externally.
An advantage of SOFC systems is that their high operating temperature facilitates close thermal integration between the fuel cell and the fuel processing system. The high temperature also allows the elimination of noble metal catalysts required by lower temperature fuel cells. However, prior art SOFC systems face challenging temperature regimes, and are disadvantaged by the degradation of cell voltages at very high temperatures under conventional operating conditions.
The lower heat of combustion of a fuel usefully defines the energy (enthalpy change of the reaction) that may be generated by oxidizing that fuel. The electrochemical energy that can be generated by an ideal fuel cell is however the free energy change of the reaction, which is smaller than the enthalpy change. The difference between the enthalpy change and the free energy change is the product of the entropy change of the reaction multiplied by the absolute temperature. This difference widens at higher temperatures, so higher temperature fuel cells inherently convert a lower fraction of the fuel energy to electrical power at high efficiency, while a larger fraction of the fuel energy is available only as heat which must be converted to electrical power by a thermodynamic bottoming cycle (e.g. steam or gas turbine plant) at lower efficiency.
Accumulation of reforming reaction products (carbon dioxide and steam) on the fuel cell anode opposes the electrochemical reaction, so that the free energy is reduced. Higher partial pressure of oxygen over the cathode, and higher partial pressure of hydrogen over the anode, drive the reaction forward so that the free energy is increased. Unfortunately, the reaction depletes the oxygen in the cathode channel and depletes hydrogen in the anode channel while rapidly increasing the backpressure of carbon dioxide as a diluent in the anode channel. Hence the free energy change is reduced, directly reducing the cell voltage of the fuel stack. This degrades the electrical efficiency of the system, while increasing the heat that must be converted at already lower efficiency by the thermal bottoming cycle.
The free energy change is simply the product of the electromotive force (“E”) of the cell and the charge transferred per mole by the reaction (“2F”), where the factor of two reflects the valency of the carbonate ion. The following Nernst relation for a SOFC expresses the above described sensitivity of the electromotive force to the partial pressures of the electrochemical reactants in the anode and cathode channels, where the standard electromotive force (“Eo”) is referred to all components at standard conditions and with water as vapor.
  E  =            E      o        -                  RT                  2          ⁢          F                    ⁢              ln        ⁡                  [                                    P                              H20                ⁡                                  (                  anode                  )                                                                                    P                                  H2                  ⁡                                      (                    anode                    )                                                              ·                              P                                  O2                  ⁡                                      (                    cathode                    )                                                  0.5                                              ]                    
Adsorption gas separation systems have been considered in the prior art for manipulating partial pressures of reactants in the fuel cell, so as to achieve higher fuel cell voltage E.
According to prior known adsorptive processes, for enriching a component A of a feed gas mixture containing components A and B, an adsorbent material over which component B is more readily adsorbed and component A is less readily adsorbed may be provided. The adsorbent material contacts flow channels in adsorbers or adsorbent beds. When the gas mixture is introduced at a feed pressure and temperature to a first end of the adsorber during a feed step of the process, component B is preferentially adsorbed and a first product enriched in component A may be delivered from the second end of the adsorber as it becomes loaded with component B. The adsorber may then be regenerated to desorb component B in reverse flow so that the process may be repeated cyclically.
Regeneration of adsorbent materials may be achieved by alternative strategies including pressure swing, displacement purge, thermal swing, or combinations thereof, according to the prior art. It has also been claimed that regeneration of a carbon adsorbent loaded with carbon dioxide may be achieved by applying an electric current in so-called electric swing adsorption.
In existing pressure swing adsorption (PSA) systems or vacuum pressure swing adsorption systems (VPSA), the total pressure of the gas contacting the adsorber is reduced (pressure swing) following the feed step, thus reducing the partial pressure of component B contacting the adsorbent, and desorbing component B to be exhausted by purging with a reflux fraction of already enriched component A. The total pressure of the gas mixture in the adsorber is elevated while the gas flow in the adsorber is directed from the first end to the second end thereof, while the total pressure is reduced in the regeneration step while the gas flow in the adsorber is directed from the second end back to the first end. As a result, a “light” product (a gas fraction depleted in the more readily adsorbed component and enriched in the less readily adsorbed component A) is delivered from the second end of the adsorber, and a “heavy” product (a gas fraction enriched in the more strongly adsorbed component B) is exhausted from the first end of the adsorber.
Alternatively, the total pressure may be kept approximately constant in the regeneration step, while component B is desorbed by a third preferably less readily adsorbed component C, which was not part of the feed gas mixture, with component C introduced in reverse flow from the second end back to the first end of the adsorbers (displacement purge), thus reducing the partial pressure of component B contacting the adsorbent, and exhausting displaced component B from the first end of the adsorbers. As a result, a first or “light” product (a gas fraction depleted in the more readily adsorbed component B and enriched in the less readily adsorbed component A) is delivered from the second end of the adsorber, and a “heavy” product (a gas mixture including the more strongly adsorbed component B and the displacement component C) is exhausted from the first end of the adsorber.
Regeneration may also be achieved by cyclically raising the temperature (temperature swing) of the adsorbent so as to reduce the adsorptive affinity for all gas species, resulting in desorption of component B which can then be purged in reverse flow by a purge stream either as a reflux of previously enriched component A or by displacement purge with a component C. Thermal swing adsorption (TSA) requires bulk heating and cooling of the adsorbent on a cyclic basis, so is limited to relatively low cycle frequencies. The heating step may be achieved by heating the purge stream before admission to the second end of the adsorbers.
According to the prior art, pressure swing and displacement purge may be combined, so that a displacement purge regeneration step is achieved at a lower total pressure than the feed pressure. When relatively low cycle frequency necessary for operation of thermal swing adsorption processes may be acceptable, thermal swing may be combined with pressure swing and/or displacement purge regeneration strategies. The distinction of displacement purge processes in the present context is that the displacement purge stream is externally provided and includes a component C that is not contained in the feed gas mixture to be separated, unlike conventional PSA or TSA processes where the purge stream is typically obtained internally as a fraction of the feed gas mixture undergoing separation.
Previously, application of displacement purge processes has been limited by compatibility of components A, B and C. Even within the context of an overall separation being achieved, some intimate mixing will take place due to axial dispersion in the adsorbers, fluid holdup in gas cavities, and leakage across fluid seals and valves. While components B and C must obviously be compatible as they will be mixed as an intended outcome of the process, cross-contamination between components A and C would also take place so as to require compatibility of those components as well.
PSA is widely applied in hydrogen purification (e.g. from syngas generated by steam reforming or gasification of a hydrocarbon feedstock, after water gas shifting to minimize carbon monoxide concentration), with components A and B representing hydrogen and carbon dioxide respectively. In that application, displacement purge using air (or any oxygen-containing gas with oxygen appearing as a component C) would in the prior art have been impracticable owing to unacceptable hazards of cross-contamination between hydrogen and oxygen.